Large amounts of white horehound may increase the risk of abnormal heart rhythms and should be avoided by people treated with drugs that affect heart rhythm. Animal studies suggest that the use of white horehound with medications that lower blood pressure may cause a larger than expected drop in blood pressure. White horehound contains glycoside compounds that act on the heart and these theoretically could affect the activity of glycoside medications such as digoxin (Lanoxin®). Theoretically, white horehound may increase the action of the hormone aldosterone on the kidneys and it may interact with some diuretic medications.

Based on animal studies, white horehound may lower blood sugar levels. Caution is advised when using medications that may also lower blood sugar. Patients taking drugs for diabetes by mouth or insulin should be monitored closely by a qualified healthcare professional. Medication adjustments may be necessary. In theory, white horehound may also interact with medications used to treat thyroid disorders such as iodine, liothyronine (T3, Cytomel®), methimazole (Tapazole®), propylthiouracil (PTU), thyroxine (T4, Levoxyl®, Synthroid®), and Thyrolar® (T4 plus T3).

White horehound may contain estrogen- like chemicals that either have stimulatory or inhibitory effects on estrogen- sensitive parts of the body. It is unclear what effects may occur in people using hormonal therapies such as birth control pills or hormone replacement therapy. Based on early animal study, white horehound may lower cholesterol or triglyceride blood levels and therefore may have additive effects with other drugs with similar actions.

Interactions with Herbs and Dietary Supplements

In theory, white horehound may lower blood pressure and may cause increased urine production. Caution is advised when using herbs or supplements that lower blood pressure or increase urination.

White horehound may contain glycoside chemicals that affect the heart and therefore should be used with caution by people taking other supplements that have glycoside ingredients. Notably, bufalin/ Chan Suis is a Chinese herbal formula that has been reported as toxic or fatal when taken with cardiac glycosides.

Because white horehound may cause diarrhea, use caution if combining it with other laxative herbs.

Animal studies suggest that white horehound may lower blood sugar levels. Caution is advised when using herbs or supplements that may also lower blood sugar. Blood glucose levels may require monitoring, and doses may need adjustment.

Because white horehound may contain estrogen- like chemicals, the effects of other agents believed to have estrogen- like properties may be altered. In theory, white horehound may interact with agents that affect the thyroid, such as bladderwrack. Based on early animal study, white horehound may lower cholesterol or triglyceride blood levels and therefore may have additive effects with other herbs and supplements with similar actions.

White horehound may interact with herbs and supplements taken to treat cough, vomiting, migraine headache, and depression; use cautiously.

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